The present invention relates to a rotatable polygon mirror to be used in an optical equipment or the like having therein a laser scanning system.
A rotatable polygon mirror to be provided in an equipment or the like having therein a laser scanning system is a prism member whose section is generally a regular polygon such as a regular octagon and whose sides are reflecting mirror surfaces. The purpose of such polygon mirror, for example, is to change an advance direction of a laser beam and thereby to scan a photoreceptor surface.
As a material of such rotating polygon mirror, an aluminum alloy or optical glass has conventionally been used. In case of the former, after being formed to be a regular polygon through machining, it has been finished to be a reflecting mirror surface through machining by means of a carbide tool, while in case of the latter, it has been finished to be a reflecting mirror surface through polishing after being formed to be a regular polygon.
In case of the glass material to be ground and polished mentioned above, it has required a great deal of time to achieve accuracy of flatness on its each side, resulting in the high cost that is ten times or more that of the aluminum alloy to be machined. Even in the case of metal to be machined, a yield rate thereof has been much lower and machining hours required therefor have been much longer than those for moldings due to the difficulty of machining caused by the curvature on the marks produced on the metal surface by a cutting tool during machining or by mold cavities in the material itself, which has resulted in a high cost also from the viewpoints of expenses both for materials and labors.
A rotating polygon mirror made of metallic material or glass material, due to its heavy weight, requires complicated and expensive driving system and control system for high speed rotation. In addition to that, the rotating polygon mirror itself requires a great deal of labor and time in aforesaid manufacturing processes for forming a highly accurate reflecting mirror surface, which has been a problem to be solved.
In view of the situation mentioned above, an attempt to form a rotating polygon mirror using synthetic resin material in place of metallic material or glass is now being made. In case of injection-molding for resin materials, however, molding pressure for resin materials can not be uniform due to the complicated form of the rotating polygon mirror, and thereby warpage and shrinkage caused by internal stress tend to be generated, which has made it extremely difficult to obtain a reflecting mirror surface that is sufficient in terms of mechanical strength and is accurate in terms of flatness.